Omnivore and vegetarians show similar body composition and skin physiology across body regions—A comparative analysis

Abstract Background Skin physiology seems to be influenced by dietary choices and body composition, although links between these factors remain poorly characterised. In the present manuscript, we elaborate on the potential relationships among food groups, body composition and skin physiology in omnivores and vegetarians. Material and Methods This cross‐sectional observational study involved 181 participants, 129 omnivores and 52 vegetarians. The main functions of the skin measured in our laboratory were transepidermal water loss, deep and superficial epidermal hydration, skin elasticity, and carotenoid content. Skin variables obtained from different body regions were made comparable by a new Proportional Skin Index calculated to respect their relative representativity. Results No statistical differences were found when comparing both groups' body composition and skin variables from different body regions, with the exception of the skin carotenoid content significantly higher in the vegetarian group (p < 0.001). Conclusion Although dietary patterns significantly differed between groups, with vegetarians consuming fewer animal‐derived products and more plant‐based foods, multiple linear regression analysis revealed no differences or association between the dietary pattern and the skin physiology. These findings highlight the need for further research to elucidate the specific impact of diet and food groups and body composition on skin physiology.

Fruits and vegetables have long been associated with a protective effect against skin aging and photoaging, attributed to their antioxidant content. 9,10On the opposite side, high glycaemic load diets have been related to exacerbated acne. 10Only a few current systematic reviews highlighted the importance of nutritional patterns to promote a healthy skin condition and preservation, allegedly influencing skin aging. 6,7,11well-known and long-acknowledged difficulty in broad skin studies is related to skin heterogenicity. 12,13Epidermis basal layer architecture greatly varies with skin region and also involves hair follicles, eccrine sweat glands and melanocytes, determining different physiological performances that also differ with race and sex, and change during our lifetime as we age.Additional complexities arise from the chemical (molecular) composition of skin related to histology and cell distribution, as well as differences in the microbiome and lifestyles, including daily (hygiene) habits. 14,15Thus, it is clear that skin morphology and related immuno-metabolo-physiology will be challenging to compare without attending to those specificities.In fact, when assessing principal skin properties in different body regions, such variables as epidermal water balance, biomechanics or pH are collected in very small areas (in most cases < 1 cm 2 ).Since an average adult skin approximately corresponds to 2 m 2 and some 15% of total body mass, 16,17 the representativity of those measurements should be questioned.[20][21] The primary objective of our study was to characterize the skin condition of two healthy groups of participants distinguished by their dietary options-omnivores and vegetarians.Here we aimed to compare major indicators of skin physiology and to identify potential relationships between food groups associated with specific dietary patterns, skin function and body composition.Secondarily, we also addressed this skin representativity issue by proposing a new 'all regions' cartography index meant to proportionally represent global skin physiological properties.

Characterization data
Information was gathered during a face-to-face interview divided into three phases-(1) general characterization of each participant; (2)   physical activity classification considering the International Physical Activity Questionnaire-Short Form 24 ; and (3) Food Frequency Questionnaire (FFQ) validated for the Portuguese population to identify the dietary intake. 25Participants assignment to each group was determined by the FFQ responses.The vegetarian group, which did not consume any type of meat, meat products, or fish, also included a few individuals following vegan dietary practices, which also exclude the consumption of any food produced by animals, including dairy, eggs and honey.The omnivore group involved all participants regularly consuming meat, meat products or fish in their regular food habits.
Body mass was measured by an electronic scale [0.1 kg accuracy], wearing light clothes and no shoes.Height was self-reported.Body Mass Index (BMI) was then calculated [BMI = Body mass (kg)/height (m) 2 ] 26 .

Morphometry (body composition and skin functional assessment)
Body composition measurements were performed after 12 h of fasting and no exercise for 24 h before measurements 27 via DXA (Lunar Prodigy™ Advance-General Electric Healthcare; Chicago, Illinois, USA).Chosen variables were Visceral Adipose Tissue (VAT) Subcutaneous Adipose Tissue (SAT) and Total Adipose Tissue (TAT).
For skin measurements, participants were allowed to acclimatize to the laboratory conditions (temperature: 22 ± 2 • C; relative humidity: 40%−60%) in a comfortably seated position for about 30 min.Participants were required to not apply any cosmetics or toiletries in the testing areas during the 24 h prior to skin assessment.

Statistical analysis
Results were expressed as mean ± standard deviation (SD) and as relative frequencies (%) for continuous or categorical variables as appropriate.The Student's t-test was used for comparisons between continuous variables, while for categorical variables, the Chi-square test was chosen whenever variables followed a normal distribution.
The association between dietary patterns, food groups and skin parameters was analysed by logistic regression.Multiple linear regression models were performed to assess the relation among different independent variables and skin descriptors.After testing the assumptions for linear regression and collinearity diagnostics, the most common variables in the models-and thus considered as possible predictor variables on skin parameters-identified by this method were dietary pattern (defined as vegetarian or omnivore), biological sex (defined as male or female), age, BMI, TAT, SAT, VAT and smoking status (defined as smoker, former smoker and non-smoker).Statistical analysis was performed using Jamovi (Version 2.2, the Jamovi Project, Sydney, Australia).The significance level was set at p < 0.05.

RESULTS
Participants were mainly non-smokers (77.3%) reporting moderate to high physical activity (75.7%).Regarding body composition, no statistical differences were found between omnivore and vegetarian groups involving body mass, BMI, waist circumference and adipose tissue-related values (Table 1).
Regarding skin physiology, no differences in the Proportional Skin Index (PSIx), TEWL, deep or superficial hydration, or elasticity were found (Table 2).Nevertheless, significantly higher TEWL in the leg and lower superficial hydration in the forehead and neck were observed for vegetarians.Statistically significant differences between groups were only observed for the skin carotenoid content, with higher values found in the vegetarian group (Table 2).

TA B L E 1
General and anthropometric characteristics of included participants according to the dietary pattern.Food group consumption significantly differed between both dietary groups, where vegetarians showed a lower consumption of milk and dairy products, meat, fish, eggs, oils, fast food and soft drinks, and a higher daily intake of pulses (Table 4).
Linear regression allowed us to assess the impact of these different food groups' consumption on skin physiology, including skin carotenoids (Table 5).Again, no significant differences were observed between groups.Multiple linear regression was further explored, as shown in Table 6.Results suggest that TEWL, deep and superficial hydration, elasticity, and carotenoids do not seem to be influenced directly by adipose tissue (SAT and VAT), waist circumference or BMI.

DISCUSSION
Vegetarian diet has been referred to improve skin health and appearance when compared to an omnivore diet by favouring barrier function, hydration and elasticity. 2,7,32The present manuscript addresses precisely these relationships recognised as major issues in nutrition physiology.Our approach is innovative, as we look deeper into poten-  1 and Tables 2-6).Some statistically significant differences between both groups were found for TEWL measured in the leg, higher in the vegetarian group (p < 0.003) and superficial hydration was significantly lower (p < 0.05) in the forehead and neck in the same group compared to omnivores (Table 2).The physiological significance of these differences cannot be determined based on the data collected in this study.
Interestingly, skin carotenoid content differences between groups were barely significant (p = 0.046).These complex compounds were recently made accessible (non-invasively) through the skin by spectroscopy.Previous research has demonstrated a direct relationship between these nutrients and lifestyles, including a healthy diet rich in fruit and vegetables. 33,34Preliminary studies on carotenoid kinetics demonstrated the sensitivity of these nutrients to stress factors such as sun radiation, environmental hazards or pathophysiological processes. 34Currently, it is accepted that a high concentration of carotenoids in the skin provides the best protection strategy against skin photo-induced aging, especially for lower-grade phototypes.Recent studies have suggested the interest of vegetarian diets in preserving a good skin condition, mostly related to the high antioxidant properties of the diets. 35,36Recently published reviews seem to confirm the wide photoprotection capacities of some of dietary components, carotenoids in particular, and the potential benefits associated with their prevalence in diet. 37,38This evidence led us to expect a better physiological performance from vegetarians´skin compared to omnivores.However, results focused on elasticity, one of the most common descriptors used in human studies, 29,28 depicted no differences between the two groups (Table 3).By further analysing the potential relationship between the dietary pattern and these skin variables by multiple regression also adjusted for sex, BMI, smoking status and physical activity, significant differences between both groups could only be found for the skin carotenoid content (Table 3).
Significant differences in food consumption emerged, as expected, between omnivorous and vegetarian groups (Table 4).Vegetarian participants showed lower consumption of milk and dairy products, meat, fish, eggs, oils, fast food and soft drinks compared to their omnivorous counterparts.Our results are in line with the current characterization of vegetarian diets, where the prominence of plant-derived foods is noted alongside the exclusion of practically all animal-origin products.The consumption of eggs, a nutrient-rich food source for proteins, including collagen and elastin precursors in particular, were expected to relate positively with skin elasticity. 38,39However, no such relationships could be found (Table 5).
Another main nutrient is water.0][41] Regarding human skin, a water supplementation study in a healthy omnivore group, adding 2 L/day for 30 days to participants' regular diets, significantly improved their superficial and deep skin hydration, as well as biomechanical descriptors. 40Recent studies confirmed the interest of dietary water in skin homeostasis and relief of dryness, roughness and irritation. 42However, no differences in total fluid daily intake between groups or any relationship with skin physiological variables were found (Table 5).
Vegetarian diets have been linked to weight loss, BMI decreases, and fat mass reduction. 36,43Our results, however, showed no association between these factors (Table 6), and we note that the mean BMI was similar for both groups (Table 1).
As far as our knowledge goes, our study is the first to look widely into these potential relationships among dietary patterns, body com- Abbreviations: CI, confidence interval; OR, odds ratio; SAT, subcutaneous adipose tissue; TEWL, transepidermal water loss; VAT, visceral adipose tissue.
of body functions and composition, and better comparisons of skin dimension.

CONCLUSION
The skin carotenoid content was the only variable significantly distinct between vegetarian and omnivorous dietary groups.Future research should further explore the specific impact of food groups on specific skin variables, eliminating potential confounding factors to refine and better understand the dynamic interplay between nutrition and skin health and its functional and esthetic preservation.
, were used to calculate one body weighted means-a Proportional Skin Index (PSIx)-for each skin functional variable measured in different regions.Relevant skin variables were obtained by noninvasive technologies and included transepidermal water loss (TEWL), a direct indicator of the epidermal 'barrier' , superficial and deep epidermal hydration, elasticity and carotenoid content.TEWL, expressed F I G U R E 1 Skin surface cartography model showing the relative percentage of each body region regarding an adult skin area of approximately 2 m 2 .

Table 3
contains the multiple linear regression analysis between the dietary pattern and skin parameters.Within the analysed sample, it was not possible to establish an association between dietary patterns (vegetarian or omnivorous) and basic skin physiology represented by TEWL, hydration and biomechanics (p > 0.05).Nevertheless, skin carotenoids were statistically associated with a vegetarian dietary pattern (p = 0.028).This association remained unchanged after adjustment for different confounders (p = 0.021).
Characterization of principal skin physiological functions in different body regions of all participants grouped by their dietary pattern.Multiple linear regression from (crude and adjusted) data relating dietary pattern and skin variables obtained from different body regions and represented in each case by the respective Proportional Skin Index (PSIx).The carotenoid content was detected in the palm of the hand (see text).Food group consumption according to dietary pattern.
Our data found no differences between the skin of omnivores and vegetarians as expressed by this Proportional Skin Index, a new descriptor here proposed to proportionally describe the same variableTA B L E 2Note: Data expressed as mean ± Standard Deviation (SD).Significance level p < 0.05 (statistically significant values in bold).p-valuesfor group comparisons between omnivores and vegetarians were tested by the Student´s t-test.TEWL, transepidermal water loss.Using each variable obtained in those five regions, one Proportional Skin Index (PSIx) was calculated corresponding to a global proportional representation of the measured variable (Forehead, Cheek, Neck = 5.5%; Hand = 3.0%; Leg = 13.0%).TA B L E 3Notes: Significance level p < 0.05 (statistically significant values in bold).Adjusted model for sex, BMI, smoking status, physical activity practice, skin type, and age.The vegetarian group was considered as the reference (REF = 1).Abbreviations: CI, confidence interval; OR, odds ratio.TA B L E 4Note: Data expressed as mean ± Standard Deviation (SD).Significance level p < 0.05 (values in bold type).p-valuesfor group comparisons between omnivore and vegetarians were tested by student´s t-test.obtained in different body sites with different area/volume representativity (Figure Multiple linear regression from (crude and adjusted) data relating consumed food by both dietary groups and skin variables.Multiple linear regression from (crude and adjusted) data relating body composition and skin variables.Note: Significance level p < 0.05.Adjusted model for biological sex, smoking status, physical activity practice, skin type, age, dietary pattern.
position, and skin physiology.Although recognizing some limitations, mainly related to (1) the sample dimension and representativity, (2) this relative scarcity of differences and (3) a new skin index not yet validated, we trust the rigor of our approach, supported by a validated FFQ, referenced assessment instruments for quantitative analysisTA B L E 5Note: Significance level p < 0.05.Adjusted model for biological sex, BMI, smoking status, physical activity practice, skin type, age, dietary pattern.Abbreviations: CI, confidence interval; OR, odds ratio; PSIx, proportional skin index; TEWL, transepidermal water loss.TA B L E 6